This application claims the benefits of Israeli Patent Application Ser. No. IL 136,685, filed Jun. 12, 2000.
1. Field of the Invention
The present invention relates generally to treatment of radioactive and hazardous wastes, and more particularly to processes for immobilizing a waste containing one or more of radionuclides, hazardous elements, hazardous compounds, and other compounds present in the waste.
2. Background of the Invention
The use of radioactive and hazardous materials in the world has led to the accumulation of a significant amount of radioactive and hazardous wastes. There is an international consensus regarding the planned disposal of these wastes by burying them in the ground in deep geological repositories. At the present time, high-level radioactive wastes are being placed in long-term storage awaiting permanent disposal. Once buried, with the passage of time, groundwater and hydrothermal solutions can make contact with the radionuclides, hazardous elements, or hazardous compounds contained in the wastes. As a result, groundwater and hydrothermal solutions can facilitate the leaching of radionuclides, hazardous elements, and hazardous compounds out of the wastes into the biosphere in which plants and animals live. In addition, even without the interference from groundwater and hydrothermal solutions, radionuclides, hazardous elements, or hazardous compounds could possibly diffuse out of the wastes, resulting in contamination of the biosphere. Therefore, improper containment of the wastes can create a significant problem.
There are a number of existing processes that can potentially reduce the leaching and/or diffusion of radioactive and hazardous wastes. The existing processes, however, have various disadvantages. For example, cementation is commonly used to immobilize low-level and intermediate-level radioactive waste. This process is undesirable because a large volume of cement is required to immobilize a small quantity of wastes. Furthermore, cement is highly susceptible to both leaching and diffusion.
The most common method of handling high-level radioactive wastes is the vitrification in borosilicate glass. This process, however, has at least the following limitations. First, the vitrification creates a relatively weak bond between the radionuclides and the components of the glass. Second, the vitrification process distributes the radionuclides throughout the entire volume of the glass matrix rather than containing the radionuclides in a more concentrated manner. Vitrification is currently being used in a number of countries including France, the United States of America, Korea, Italy, Germany, the United Kingdom, Japan, Belgium, China, and Russia.
The present invention comprises one or more waste processing stages to immobilize wastes which contain radionuclides, hazardous elements, hazardous compounds, and/or other compounds such as waste oxides that are present in the waste. Each of the waste processing stages creates a barrier that immobilizes the waste. The preferred embodiment of the present invention creates a stable thermodynamic system comprising a number of physicochemical barriers. Each of these barriers decreases leaching and diffusion of radioactive and hazardous materials into the environment.
The first barrier is created by integrating the radionuclides, hazardous elements, hazardous compounds, and/or other compounds found in the waste with immobilizing minerals. Integration of the waste with the immobilizing minerals can be performed using one of several methods, including activation-absorption-transformation, synthesis, and solid state reaction. The product of each of these methods is hereinafter referred to as a waste integrated mineral or the first barrier. The process of creating this first barrier results in a loading of radionuclides, hazardous elements, hazardous compounds, and/or other compounds found in the waste that is two weight percent or greater. As shown in Example 1 described below, an immobilizing mineral can incorporate more than 75 weight % loading of thorium oxide. As shown in Example 5, loading of waste oxide can be as high as 100% weight.
The invention preferably includes a second barrier which is an effective covering comprising a layer of non-radioactive and non-hazardous material that covers the first barrier. The second barrier has a mineral composition that is the same as or similar to the immobilizing minerals of the first barrier. There are at least two ways to create the second barrier. First, the second barrier can be obtained by an overgrowth procedure that yields a crystalline material upon the surface of the first barrier. The thickness of the overgrowth is at least two microns, preferably at least 50 microns. Second, the second barrier can be obtained by sintering that yields a polycrystalline covering over the first barrier. The second barrier by sintering is at least several millimeters thick. Due to the high temperature caused by (i) the incorporation of radioactive materials into the immobilizing minerals, and (ii) the geothermal gradient, the second barrier will grow continuously (albeit at a slow rate) even after burial at the disposal site, thereby increasing the safety of the second barrier.
The invention preferably includes a third barrier, which is a surrounding matrix comprising either rock or glass that encapsulates the second barrier. Additional waste containing radionuclides, hazardous elements, hazardous compounds, and/or other compounds present in the waste can be mixed with the rock or glass, thus increasing the total amount of waste that is immobilized. In embodiments in which the second barrier is not present, the third barrier surrounds the first barrier. The rock or glass utilized to create the third barrier preferably contains some of the same components as the immobilizing minerals so that the immobilizing minerals can be in equilibrium with the rock or glass. The equilibrium state adds to the strength of the third barrier since the components of the immobilizing minerals will not readily diffuse to the surrounding matrix.
In addition to being barriers against leaching, these three barriers can effectively reduce the diffusion of the radionuclides and the hazardous elements or compounds to a negligible level.
The radioactive and hazardous wastes can be further immobilized by ensuring that the surrounding matrix comprising one of rock and glass either has a composition that is the same as or similar to the composition of the indigenous rock at the disposal site. It is preferable that the rock or glass has at least one component that is in common with components of the indigenous rock. In this case, the surrounding matrix will be in equilibrium with the groundwater or local hydrothermal solutions at the disposal site when the groundwater or local hydrothermal solutions are saturated with components of the indigenous rock. The rates of leaching or diffusion of radionuclides and hazardous elements or compounds out of the surrounding matrix are negligible in the equilibrium state. In effect, the equilibrium state can be considered as a fourth barrier against leaching and diffusion.